Another 25 people were wounded, many of them critically, in the bombing in Baluchistan's Mastung district, said Tufail Ahmed, a local political official.

The blast destroyed the bus and damaged a second bus carrying more Shiites.

Mr Ahmed and a passenger on the second bus, Mohammed Ayan Danish, said the attack was carried out by a suicide bomber who "rammed a small car into the first bus, which contained 43 pilgrims".

But Akbar Durrani, the home secretary in Baluchistan, said the explosion was caused by a car packed with explosives that was parked beside the road and detonated by remote control.

Pakistan has experienced a spike in killings over the last year by radical Sunni Muslims targeting Shiites, whom they consider heretics. The violence has been especially pronounced in Baluchistan province.

The pilgrims who were targeted were headed to Iran, a majority Shiite country that is a popular religious tourism destination, Mr Ahmed said.

Shiites make up around 15% of Pakistan's 190 million people. They are scattered around the country, but Baluchistan has the largest community, mainly made up of ethnic Hazaras, easily identified by their facial features, which resemble those of Central Asians.

Sunni extremists have long carried out attacks against Shiites in Pakistan. The sectarian campaign has stepped up in recent years, fuelled mainly by the radical group Laskar-e-Jangvhi, aligned to Pakistani Taliban militants headquartered in the tribal region.

More than 300 Shiites have been killed in Pakistan this year, according to Human Rights Watch.